Airplane Barometer

PJ’s on the plane… it was a pre-dawn flight out.

Here’s a fun little quickie I came up with while awaiting the big southwest adventure trip. Our first flight to Albequerque had a stopover in Chicago- while at the intial gate I came up with this plan for an improv science project (like we do). It’s cheap (or free), quick, easy, and educational.

We used a clear plastic apple juice bottle, a clear plastic straw, and a wooden coffee stirrer. You will want some kind of clear bottle that can flex a little bit that has a cap you can twist down extra-tight.

The goal is to get a bubble of air trapped in the straw that you can watch as you change altitude/pressure. An ideal shape would resemble an eye dropper (or conventional themometer) such that a sealed bulb of gas exists in the top and the bottom is submerged but open so that as the gas volume changes density it will grow and shrink while confined within the straw.

The particular apple juice bottle was pretty ideal because it was A) Very clear, with flat sides & labels you could carefully peel off giving an unobstruted view. B) Because it had (for some reason) a big lid with oversized burly threads allowing for an extra tight screw-down.

Step one is to fill your bottle with water, nearly to the top.

Step two is to insert your wooden coffee stirrer in one end of the straw. This step is actually somewhat optional but if I had it to do over, I’d also have used some writing utensil to make some even gradient marks along it. You will put it into the straw, and break it if needed so that it leaves a bit of space at the top. Its purpose is primarily just to take up space, so that the larger air pocket is at the top while the reduced air passage along the stirrer more dramatically illustrates the changing gas density. It’s the same reason a thermometer has a bulb at one end and a very narrow passage to view the mercury. All that said, we did end up exceeding the volume of our straw resulting in a few bubbles and killing our reliability/accuracy, but this is a kids science experiment so who cares?

Step three is just bend the straw about an inch from one end (you can use a white or maybe other color one but try to get a clear straw if you can for best visibility). You want a hard kink that blocks air and allows you to sink the straw into the water enough to get the air volume at the top, and then with a bit of straw still hanging out of the bottle top, turn the cap on very tightly. You wil need to turn it tight enough to not only seal the bottle but also to crush and seal the straw top, so it will take a bit of extra effort to get that and not strip the threads of a disposable bottle.

That’s pretty much it. Most airlines have a flight tracker you can view for free as well- so if you want to calibrate this; you (sort of) can. Ours maxxed out at (I think) 42,000ft, but we’d lost some bubbles by then. I do know that cabins are pressurized to some degree, so I thought it was interesting to get an idea of about how much. I’d read that isobaric systems typically max out at 6-8000ft, but ours seemed to be a dynamically proportional pressurization. You could take this to the next level with a little geometry & math but still would probably need a real baro to get some useful data. Maybe next time.